A. Field of the Invention
The invention generally concerns microporous polymeric membranes for selective recovery of desired metals and methods of making and using the same.
B. Description of Related Art
The demand for precious metals, such as gold, is high and continues to grow. Gold and other precious metals are widely used in electronics, jewelry, catalysts and medical applications. In addition, emerging technologies, such as nanoelectronics and microelectromechanical systems (MEMS), generate more pressure on the already high demand.
Given the number and variety of articles that contain precious metals, these discarded articles can be a viable source of precious metals. For example, cellular phone scrap contains around 200 g of gold per ton of scrap, while the gold present in the gold ores used in mining contains between 5 and 30 g of gold per ton of ore. Ogata, T. and Y. Nakano, “Mechanisms of gold recovery from aqueous solutions using a novel tannin gel adsorbent synthesized from natural condensed tannin,” Water Research, 39(18): 4281-4286 (2005).
To recover the gold present in electronic scrap, for example, the scrap is dissolved in an acidic solution. The gold is then recovered from the acidic solution. The current techniques used to recover the gold include precipitation, ion exchange, solvent extraction or adsorption into a solid adsorbent. These current recovery and recycling technologies present various disadvantages such as slow kinetics, low selectivity, the requirement of large amounts of organic solvents, and/or irreversible adsorption of metal.
For example, the main disadvantages of solvent extraction methods include the large amounts of organic solvents required, the loss of residual organic solvent to the aqueous phase, and the difficulty of implementing into a continuous process. Some commercial examples of solvent extraction include using Dibutylcarbitol (DBC) and methyl isobutyl ketone (MIBK) as extractants for Au(III) in HCl solutions. However, these extractants exhibit relatively low selectivity, and the subsequent recovery is usually done by reduction of the organic to form metallic gold. US Patent Publication No. 2012/0234138 presents a method to use DBC for very diluted gold solutions (10 ppm or less). Another solvent extraction method is presented in US Patent Publication No. 2012/0228151, describing a process to efficiently recover gold and palladium from acidic solutions using dithiobiuret derivatives.
With regard to ion-exchange resins to recover the gold from acidic solutions, in most cases, the resin needs to be destroyed to recover the gold, as it is either irreversibly adsorbed or in-situ reduction takes place. U.S. Pat. No. 5,028,260 describes a process in which the acidic gold-containing solution is oxidized with hydrogen peroxide to guaranty that all the gold ions are in the +3 oxidation state and finally contact the solution with a polymeric acrylic ester ion-exchange resin to selectively adsorb gold. Other drawbacks of ion-exchange resins include the necessity to oxidize the solution before contacting it with the resin, the low capacities of the resins, and the need to operate at slow fluxes to achieve good adsorption.
Solid adsorbents are a good option for gold recovery because they are easy to handle and tend to produce less amount of waste compared to solvent extraction. However, known solid adsorbents for gold recovery present either slow kinetics of adsorption, low capacity, or difficult elution of the gold. The most common solid adsorbent for this application is activated carbon. The disadvantage of using activated carbon as the adsorbent is the complexity and cost of the regeneration process. During the recovery cycle, some carbonates will precipitate plugging the pores of the carbon, and if there are traces of organic materials, they can deactivate the gold adsorption properties. For this reason carbon is treated with a mineral acid solution and roasted. During this process, a significant amount of carbon can be lost. Several gold adsorbents can be found in the literature with high maximum uptakes but slow kinetics, as most of them take several hours to reach equilibrium. Some examples of these adsorbents are cross-linked paper gel with a maximum uptake of 5.05 mmol/g and a time to reach equilibrium of 32 hours, tannin gel with a maximum uptake of 40.62 mmol/g and more than 300 hours to reach equilibrium, and dimethylamine-persimmon waste with a maximum uptake of 5.63 mmol/g and 5 hours to reach equilibrium.
In order to supply the ever increasing demand, new metal recovery and recycling technologies need to be developed.